Congrats on your purchase. I hope you get many years of enjoyment from her. Sounds like that bottom was pretty fouled if it was costing you 300 RPM on the top end. Please keep us posted as you work the props. I'm still very curious on what the original prop specs were compared to your current 26by27 four blade. Did a previous owner make a prop change? Or perhaps these are the original but the boat has gained weight over the years as most are prone to do?
late to the thread but on mine the props with 420 yanmars were 28x28. Still running the same props with the 450Cs and could swing a 30 on paper but didnt mess with it.